Using Platform Config Keys in Extensions
If you create an extention using the extension generator tool (https://generator.tllab.co.uk/) under General tab fill in the required fields and click on the Next button.
Under Common tab check Use platform config keys option and click on Next button.
Under Advanced tab click on Generate button to generate the zip file with the extension.
If we open the Setup file for this extension we will see that the only difference between platform config keys and config keys is one additional line for platform config keys.
Then unpack this zip file in the required place and insall this extension from the admin area.
While installing this extention you can see that the platform settings are applied to this extension and they include the information about the config keys. If there are more than one platform these settings will be applied to each platform separately.